Multiple input, multiple output (MIMO) is one technology used in wireless communication systems to meet the increasing demand for high speed data. Generally, MIMO systems employ multiple antennas at the transmitter and the receiver to take advantage of the multipath channel to increase system capacity. MIMO has become an essential element of wireless communication systems including wireless local area networks (WLANs) based on the IEEE (Institute of Electrical and Electronics Engineers) 802.11 family of standards, Long Term Evolution (LTE) networks, and Worldwide Interoperability for Microwave Access (WiMAX) networks.
In fifth generation (5G) and next generation (NG) wireless communication systems, massive MIMO involving large-scale antenna arrays and beamforming will be used to combat severe path loss expected at higher frequencies. In massive MIMO systems, the base station (BS) or access point (AP) may be equipped with hundreds of antenna elements. In conventional MIMO and beamforming systems, each antenna element uses a separate radio frequency (RF) chain and baseband precoding schemes. Providing separate RF chains for massive MIMO systems results in increased hardware costs and power consumption.
Savings in hardware costs and power consumption can be achieved by having less radio frequency (RF) chains than the number of antennas. This principle is employed in the IEEE 802.11ay standard, where the number of RF chains in the base station or access point can be as low as 8, while the number of antennas can be as high as 256. In this case, it is not possible perform digital beamforming across all antennas. Rather, the digital beamforming is performed across the RF chains, and analog beamforming is then used to map the output of the RF chains to the antenna elements. This concept, referred to as hybrid beamforming, is a hybrid between digital beamforming and analog beamforming.
One aspect of hybrid beamforming is choosing analog phases that are well adjusted to the physical channel (i.e., result in an analog beam that “points” toward a desired user equipment (UE) or UEs. For this reason, as in the IEEE 802.11ad and future IEEE 802.11ay standard, there is a downlink (DL) training phase associated with the analog beamforming part of the hybrid beamforming method, in which different analog beams are transmitted by the base station and received with analog combiners at the UE (receive analog beamforming). In this way, the base station and UE can learn the best transmit and receive analog beams, which are then used during downlink transmission and on which digital beamforming is applied on top. Typically, as in 802.11ad, the analog beamforming training is performed with a codebook-based method, where the base station and UE are equipped with finite codebooks from which the beams are chosen.